1. Field of the Invention
The invention relates generally to the field of seismic air guns used in marine seismic surveying. More particularly, the invention relates to systems for monitoring certain operating parameters for such air guns to verify performance and to diagnose operating faults.
2. Background Art
In marine seismic surveys, a seismic energy source is used to generate seismic energy in the form of acoustic pulses or waves in a body of water such a lake or the ocean. The seismic energy travels downward in the water, through the water bottom, and through the Earth formations underlying the water bottom. Part of the energy passing through the Earth formations underlying the water bottom is reflected upward therefrom at acoustic impedance boundaries in the Earth formations. The upward traveling energy is detected by sensors such as hydrophones towed in one or more streamer cables disposed near the water surface, or by sensors disposed in cables along the water bottom. The sensors convert the detected energy to electrical or optical signals. The electrical or optical signals are then conditioned and interpreted to provide information both as to the composition and the structure of the various subsurface Earth formations. Such information is used particularly to determine the possibility that such Earth formations may contain mineral deposits such as hydrocarbons.
Several different types of seismic energy sources have been used in the past to produce seismic energy in a form required in marine seismic surveys. For example, explosives have been used as a marine seismic energy source, but explosives are dangerous and are considered ecologically unacceptable for such purposes. Another type of marine seismic energy source, called a gas gun, includes detonating combustible gases in a chamber and then expelling the resultant gas charge into the water to produce the seismic energy. Certain risks are associated with the use of such gas guns, and as such gas guns are not widely used.
The most frequently used marine seismic energy source at present is known as an “air gun.” In an air gun, a charge of compressed air or inert gas under high pressure, on the order of. 2000-6000 pounds per square inch (130 to 400 bar), is supplied to a chamber within the gun. When the air gun is actuated or “fired”, a valve rapidly opens to discharge the pressurized air or gas from the chamber into the water. The valve then closes and the chamber is repressurized with air or gas from a source such as a compressor. Such firing can be repeated as often as desired and within the capacity of the source to repressurize the chamber. The air or gas discharges in a bubble or plurality of such bubbles in the water.
A single air gun produces a seismic pulse having acoustic energy content with respect to frequency (the source “signature”) related to a complex pressure interaction between the air bubbles and the water that causes the bubbles to oscillate as they float toward the water surface. Such interaction can produce extraneous bursts of seismic energy following the initial energy burst. The amplitude and periodicity of these bubble-generated extraneous bursts depend on, among other factors, the depth of the gun in the water and the size of the pressurized air chamber in the gun. It is therefore known in the art to use an array of air guns having various different chamber sizes, and firing such guns contemporaneously or in a preselected timing sequence. Such firing of an array of air guns provides several advantages over firing a single air gun. First, the total amount of energy being imparted into the Earth's subsurface for each seismic “shot” is increased. In addition, the different chamber sizes for the various guns will produce different bubble responses, causing the bubble responses to tend to cancel each other. The directivity of the energy source toward the water bottom can be improved, because other than directly below the source array, some frequencies in the seismic energy will be attenuated by the spatial distribution of the guns in the array. Design considerations for air gun arrays to attain certain spectral and directivity characteristics are disclosed in U.S. Pat. No. 4,064,479 issued to Ruehle, and in U.S. Pat. No. 4,382,486 issued to Ruehle, for example.
In order to obtain the characteristics of an air gun array as described above, each air gun in the array must operate correctly. Any guns which leak air from their charge chambers so as to have lower than selected pressure therein at firing, any air guns which do not fire correctly so as to have the selected acoustic “signature” therefor, or air guns which “auto fire” or actuate other than at the selected time, or do not actuate at all, can all cause the acoustic energy content and directivity of the air gun array to be different than expected. Further, air leaks from one or more guns may introduce noise in the water that reduces the quality of the detected seismic signals.
It is known in the art to measure pressure in an “operating chamber” of a particular type of air gun in order to determine the precise moment in time at which any particular air gun fires. The operating chamber as known in the art is a part of the air gun which actuates a valve to release the high pressure air charge. Due to manufacturing tolerances and variations in the electromechanical response of the actuating control elements of typical air guns, there may be an indeterminate delay between transmission of an electrical actuation signal from a source control device and the actual discharge of compressed air or gas from the air gun. Pressure measurement within the firing mechanism of an air gun can be used to determine the actual firing moment. An arrangement for making such operating chamber pressure measurement and how to determine the firing instant are disclosed in U.S. Pat. No. 4,210,222 issued to Chelminski et al. Measuring pressure in the operating chamber of the air gun has proven useful in correcting for actuation time variations in air guns so configured. However, such pressure measurement has not found application beyond that described in the '222 patent.
What is needed is a system for monitoring performance of air guns to detect defective operation thereof, including but not limited to leaks, improper discharge of compressed gas and incorrect charge pressure.